In the past, holographic displays, especially those intended for artistic purposes, have included a hologram mounted to a wall which is illuminated by a source that is precisely spaced from and angled above the hologram such that a virtual image exists either behind or in front of the hologram. For museums, art galleries, or in-home display of the art work, such constraints on the ability to view the display limits the availability of such art forms to the public. In the past, such art works have generally been limited to display in areas in which a plain wall is in back of the hologram so that the virtual image, if in fact viewed, is viewed against a plain background. Moreover, since the public is used to focusing at the art work itself as opposed to focusing at infinity, the result is that some members of the public do not even see the virtual image produced by the hologram. This is because their normal focus is at the surface of the hologram itself and not in front or beyond. Additionally, precisely locating an illumination source is difficult because of the requirement of locating the illumination source away from a wall or at a point where there is no existing building structure.
Nevertheless, virtual image holograms are useful in so-called "heads up" displays in which the hologram is mounted to a windshield either of a motor vehicle or an aircraft, and in which the image exists at infinity. It will be appreciated that it is the purpose of these displays to display the indicated data at infinity, which is where the individual normally has his eyes focused. The patents to Gaylard E. Moss describe such virtual image holograms. These patents include U.S. Pat. Nos. 4,790,613, 4,737,001, 4,807,951 and 4,795,223. Moreover, in addition to only virtual images being supplied by the Moss patents, these patents involve edge illumination of holograms, with the hologram running completely down the windshield to the point at which light is injected into the edge of the windshield. In one embodiment, illumination is provided through the use of light pipes or fiber optics.
As mentioned, such virtual image holograms are neither convenient nor usable in the gallery or art work display setting. Moreover, since they are designed for viewing in a fixed position, as by the operator of the vehicle or aircraft, such holograms are not required to provide the three-dimensional quality for which holography is best known. In contradistinction to the virtual images produced by Moss real holographic images at the hologram permit the observer to walk part way around the image produced by the hologram such that the image presents different aspects when viewed from different angles. For instance, holograms depicting statuary are desirably real image holograms which give the observer the appearance of the statuary being at a position where the observer can at least partially walk around it so as to observe it from a number of angles.
It will also be appreciated that the Moss patents involve monochromatic images in that the thickness of the hologram is typically on the order of 30 microns for the Moss process. Holograms of this thickness filter the light and limit its wave band. They are thus incapable of producing multi-color images.
Another virtual image hologram is illustrated in U.S. Pat. No. 4,643,515 issued to Juris Upatnieks. This holographic technique provides a monochromatic virtual image at infinity behind an edge-lit hologram. Moreover, as can be seen from this patent, the hologram extends directly to the illuminated edge of the carrier. These constraints militate against the utilization of this type of hologram for museum or gallery use. It will also be appreciated that this Upatnieks patent is one in which a compact display is not possible due to reconstruction involving a laser positioned at a distance from the hologram.
Since a multi-color display is desirable for artistic purposes, white light illumination is preferred and wideband diffraction is a prerequisite. Note that the Upatnieks system produces images which contain both vertical as well as horizontal parallax, such that they would become blurred if illuminated with a white light source, due to spectral smearing. By way of further background, another Upatnieks patent, namely U.S. Pat. No. 4,223,975, details the process of the reconstruction of a virtual image.
In none of the above-mentioned patents is the image at or near the plane of the hologram. In the past, there has been a transition from looking through the hologram at a virtual image floating behind the hologram to looking at a real image that is at or near the plane of the hologram producing the image.
Although not edge-lit, one type of real image produced at or adjacent the hologram is discussed in U.S. Pat. No. 4,512,624 issued to Peter Nicholson. In this patent, a two-step process is utilized for producing a hologram which is reconstructed with a diverging beam of white light. For a compact display, the hologram should be edge-lit with the diverging reconstruction beam formed from a source located very close to the hologram. This is only possible in a two-step process with an impractically large f/0.5 converging lens. For practical f/3.0 converging lenses, the result for Nicholson is that the reconstruction source must be located at such a distance from the hologram that the source must be outside of a practical display housing or box.
Moreover, the elliptical mask utilized in forming the first hologram in the Nicholson patent is wholly ineffective in preventing deblurring for white light illumination. Given the ratios of 1:3 in this patent, and given a horizontal dimension to the hologram of a couple of feet, then the oval aperture would be approximately eight inches in the vertical direction. This would have virtually no effect to deblur a white light hologram, with the Nicholson deblurring being limited to, at most, a few inches from the hologram plane.
Deblurring is a major problem with respect to white light illuminated holograms because the individual color components are diffracted to different locations. Deblurring techniques for white light illuminated multi-color holograms are discussed in U.S. Pat. Nos. 4,623,215 to Kaveh Bazargan; and, 3,603,668 to Dominick John DeBitetto. U.S. Pat. No. 3,754,808 to Burton Ross Clay, et al, involves a hologram recorded with coherent energy of a predetermined wavelength, with the hologram being read out with broad banded energy, and with a predispersion diffraction grating utilized for deblurring. The U.S. Pat. No. 3,633,989 to Stephen A. Benton describes a narrow horizontal slit technique for limiting blurring by eliminating vertical parallax. This reduces information content in an effort to make a low bandwidth stereoscopic hologram of the subject, with a narrow horizontal aperture of a couple of millimeters being the means for limiting the vertical parallax.
A further Benton patent, namely U.S. Pat. No. 3,944,322, is primarily directed to the utilization of a flexible polarizing sheet for eliminating background light which would otherwise degrade the quality of the stereoscopic image.
More importantly, U.S. Pat. No. 4,498,729 to Benton describes a hologram production technique in which the slit technique of the aforementioned Benton patent is utilized in a three-step hologram recording process. In this three-step process, a diffraction grating is used for making an achromatic hologram. The method described in this patent refers to the steps of a making a monochromatic hologram on a first photographic plate, making a diffraction grating by exposing a second photographic plate to a series of co-linear point sources of mutually coherent monochromatic light, developing and bleaching the exposed plate to produce the diffraction grating, making a second hologram by exposing a third holographic plate to an image from a narrow elongated strip of the first hologram with the diffraction grating in the optical path, and making an achromatic hologram by holographically recording the image produced by illuminating the second hologram with monochromatic light on a fourth photographic plate. All of the holograms are recorded on photographic films or on thin plates with photographic emulsions, with this three-step process resulting in a hologram which when illuminated with white light produces an image in which colors are missing, i.e. a black and white image is produced. This diffraction grating is what is responsible for the production of the black and white hologram image, with the result being the provision of a neutral tone for viewing. While exceptionally sharp achromatic images are produced by this white light transmission holographic technique, its use in the artistic world is limited due to its achromatic nature, when color is desired, or due to the complexity of the method. Note that no edge-lit technique is described in this patent. Moreover, this patent does not teach the mounting of the final relatively thin hologram on a transparent block or support plate, as is required for edge-lit systems. Nor is any elongated white light source taught.